File content as of revision 2:c7229eac380b:

#include "mbed.h"

Serial pc(USBTX, USBRX);


const int frequ = 500; //unit kHz
const char dev_addr = 0x00; //1u

char get_half_byte(int *flag) {
    char cur_char;
    cur_char = pc.getc();
    if ( cur_char >= '0' && cur_char <= '9' ) {
        cur_char = cur_char - '0';
        *flag = 1;
    }
    else if ( cur_char >= 'a' && cur_char <= 'f') {
        cur_char = cur_char - 'a'+10;
        *flag = 1;
    }
    else if( cur_char >= 'A' && cur_char <= 'F') {
        cur_char = cur_char - 'A'+10;
        *flag = 1;
    }
    else *flag = 0; 
    return cur_char;
}   

void i2c_write() {
    char reg_ptr = 0x55;
    char reg_data = 0xaa;
    char temp;
    int flag1 = 0;
    I2C i2c(p9,p10);
    //Read Register Pointer
    pc.printf("\n\n\r----------------------\n\n\rYahoooo! Lets program a Register");
    while (!flag1) {
        pc.printf("\n\n\rEnter Register Address in hex (0 to f): ");
        reg_ptr = get_half_byte(&flag1);
        if (flag1==0) pc.printf(" Invalid Chracter!! No worries, let us try again");
    } 
    //Read Register Data
    flag1 = 0;
    while (!flag1) {
        pc.printf("\n\n\rEnter Data in hex (00 to ff): ");
        reg_data = get_half_byte(&flag1); 
        if (flag1 == 1) {
            reg_data = reg_data << 4;
            temp = get_half_byte(&flag1);
            reg_data = (reg_data | temp);
        }
        if (flag1==0) pc.printf(" Invalid Chracter!! No worries, let us try again");
    }                  
    pc.printf("\n\n\rDevice Address (fixed) = 0x%2.2X ; Register Pointer = 0x%2.2X ; Register Data = 0x%2.2X ",dev_addr, reg_ptr, reg_data);
    pc.printf("\n\n\rContinue Write? (y/n) : ");  
    temp = 0;
    while ( temp!='y' && temp!='n') {
        temp = pc.getc();
    }
    if (temp == 'n') {
        pc.printf("\n\n\rAborting Register Write :(");
        return;
    }
    i2c.frequency(frequ*100);
    i2c.stop(); //add a stop after hot-read
    i2c.start();
    flag1 = i2c.write(dev_addr);
    if (flag1 != 1) pc.printf("\n\n\rNo Ack for dev addr :(");
    flag1 = i2c.write(reg_ptr);
    if (flag1 != 1) pc.printf("\n\n\rNo Ack reg pointer :(");    
    flag1 = i2c.write(reg_data); 
    if (flag1 != 1) pc.printf("\n\n\rNo Ack data :(");           
    wait_us(5);
    i2c.stop();
    pc.printf("\n\n\rYeahhh! Register Write Complete\n\n\r---------- ");      
    wait_us(5);
    i2c.~I2C();   
}  

int main() {
    char current_char=0;
    pc.printf("\n\r---------------------------------\n\n\rReady to have fun!!\n\n\rEnter a character- 'w' to perform write operation: ");
    while(current_char!='w') {
        current_char=pc.getc();
        //pc.putc(current_char);
    }
    current_char=0;
    while(1) {
        i2c_write();
        wait_us(5); 
    }
}